A garda spokesperson told independent.ie this morning that Mr Callinan is to step down "with immediate effect".

The move has sent shockwaves through government circles and took most ministers by surprise.

“I was certainly surprised,” Mr Martin said.

“It is very difficult for him... I am sad for him.”

Mr Martin described this morning’s news as ‘the latest depressing chapter in a long saga’.

“Any criticism [Mr Shatter] he is handed out, he gets adversarial very quickly,” he continued.

“He finds it very difficult to apologise and he finds it difficult, if not impossible, to say ‘I got this one wrong’.”

A spokesperson for the Justice Minister said that no comment would be made, and the Taoiseach's spokesperson also said that no comment would be made until after today's cabinet meeting.

Speculation began to mount this morning about the Commissioner's position after every single cabinet minister, including Taoiseach Enda Kenny,

avoided the media ahead of their weekly cabinet meeting in government buildings.

The Herald can reveal that a series of high-level meetings involving ministers took place as late as last night.

Mr Kenny held separate discussions with both Mr Shatter and Mr Varadkar in a bid to diffuse the crisis that has engulfed the coalition.

Mr Varadkar and Mr Shatter also held an informal conversation in the corridors of Leinster House, sources say, during which Mr Varadkar said he was not backing down on his demands for Mr Callinan to withdraw the ‘disgusting' remark.

The relationship between two of the government's most senior ministers is now extremely fraught. Taoiseach Enda Kenny is understood to be desperate to calm tensions within Fine Gael.

Meanwhile, whistleblower John Wilson has said he takes “no pleasure” in the shock move, insisting that the Commissioner's position was untenable.

“Martin Callinan has done this country some service throughout his long career. But his position had become untenable and his decision to resign was the correct one,” he said.

But Independent TD Mick Wallace says he was “not shocked” by the move.

“I suppose the big question now is whether Minister Shatter can stay in place,” he said on RTE Radio, immediately turning the spotlight on the justice minister. He said Mr Shatter was a “disaster” in the whole area of policing.

In Mr Callinan’s statement, issued shortly before lunchtime, he did not directly refer to the whistleblower controversy but said that he decided to resign from his post because he felt the recent controversy was proving to be a distraction from the important work being carried out daily by the force on behalf of the citizens of the State in an independent and impartial manner.

He said he believed his decision to step down now was in the best interests of the force and his family.

Mr Callinan said he had great confidence that the delivery of an excellent policing service by excellent people would continue as it had done since the foundation of the force.

Having joined the Garda in May 1973, he said it had been a great honour and privilege to have spent nearly 41 years as a member of “this tremendous organisation”, serving the people of Ireland.

Reacting to the resignation, the Association of Garda Sergeants and Inspectors (AGSI) secretary John Redmond said "the imminent review of An Garda Síochána is timely and should be prioritised by Government and Garda management as a matter of urgency following the resignation..."

"The Garda organisation has come under the spotlight recently. Some of the reports and criticisms have impacted greatly with morale seriously diminished within the organisation. All of this when our members are trying to deliver a policing service in difficult times.

"As you know we are in the process of undertaking a review of the Garda organisation as part of the Haddington Road agreement. Now is the opportune time to expand the review of the Garda organisation to include oversight and accountability and indeed to look at service provision generally by the Garda organisation. The public have expectations of what service they want and yet this review is not engaging globally with all interested parties and stakeholders."